President Biden’s plan in marking January 6 with a campaign speech near Valley Forge is to cast President Trump as a threat to the democracy secured by the Continental Army. Should Mr. Biden believe that, though, what would he do if Mr. Trump were to win the election? 

It’s not my intention here to predict a victory by Mr. Trump. With polls showing him leading, though, a second term for him is not out of the question. Mr. Biden, with an all-time low 34 percent approval in last month’s Monmouth University poll, has responded by saying his opponent is not just anathema to the Spirit of 1776 but equal to Adolf Hitler.

The gambit may dump the choice between dictatorship and democracy into the incumbent’s lap. Ever since January 6, 2021, when “Mr. Trump and his acolytes” made an effort to “stop the electoral vote count,” as the Sun described it the other day, Mr. Biden has campaigned on “MAGA Republicans” being a bayonet pointed at the heart of the republic. 

Last month’s AP poll found that 62 percent of Americans share the fear that democracy may be at risk depending on November’s results. They see “government of the people, by the people, for the people” — a quote by President Lincoln that Mr. Biden has invoked — hanging in the balance.

However, Mr. Biden’s claim to be democracy’s lone defender in the race has failed to erode Mr. Trump’s support. In response, the Biden-Harris campaign has ramped up the Nazi parallels, rhetoric that may undermine the republic rather than save it.

On December 20, the Biden-Harris HQ account on X tweeted a graphic titled, “Trump Parrots Hitler,” pairing the two with quotes cobbled together from single words or short phrases absent the context Mr. Trump gave them.  

One quote puts phrases of three and two words by Mr. Trump  into an invented sentence and isn’t given any Hitler corollary. “My political opponents ‘within our country,’” it read, “are ‘far worse’ than the dictators of Russia, North Korea,” a statement akin to what Mr. Biden’s team is saying about their foe.

Politico reported last month that Mr. Biden’s campaign has made comparing Mr. Trump to Adolf Hitler “routine.” Politico counts four instances in which they’d done so in the preceding six weeks. The label has long been used against opponents. When an incumbent president swings that brickbat, though, it raises the stakes to a dangerous level.

In the decades since World War II, many individuals have been compared to Hitler and the Italian fascist, Benito Mussolini, sapping what ought to be shocking charges of their sting. Like “Nazi,” the labels have become banal political rhetoric, as harmful to those using it as to those targeted.

President Franklin Roosevelt, far more popular than Mr. Biden, pulled off the insult as an incumbent. In 1940, five years before the Nazi dictator’s death, FDR accused his Republican opponent, Wendell Wilkie, of employing “Hitler tactics” on the dubious grounds that both repeated points for emphasis.

That one instance aside, FDR didn’t frame 1940 as a choice between himself and an American dictator. He could have accepted Wilkie’s victory, never having laid the prospect of a Fourth Reich before the electorate only to see them choose it. When Mr. Wilkie lost, he made it his business to help FDR win the war.

If Mr. Trump wins, what will Mr. Biden do? The 46th president will have two choices, neither attractive. He could repudiate all the Hitler barbs as, to use one of his favorite terms, “malarky.” This would be a humiliating retreat but ensure the peaceful transition of power required by the Constitution that he lambasted Mr. Trump for denying the nation in 2021.

The malarky option would be “what democracy looks like.” Yet choice B — refusing to surrender the White House to a man he believes would be a dictator — would be undemocratic, not to mention a repeat of Mr. Trump’s attempts on that infamous January day.

To avoid the Hobson’s choice should Mr. Trump win, Mr. Biden can limit his remarks at Valley Forge to policy and have his campaign do the same. He can let surrogates beat up the Republican over rhetoric they see reflecting dictatorial impulses, and do a service to democracy by keeping his fingerprints off the Nazi brickbat.

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Correction: Wendell Willkie was the Republican nominee for president in 1940. An earlier version misstated his title.